A high-frequency transmitter used for performing radio communications includes an output circuit configured to amplify an output signal into desired transmission power. The above-described output circuit includes an amplifier amplifying the output signal, and the amplifier includes a combination of a variable gain amplifier (hereinafter referred to as a VGA) amplifying a baseband signal and/or a low-frequency signal and a VGA amplifying a high-frequency signal obtained by up-converting the frequency of a signal output from the VGA, so that a large linear variable range is achieved.
The above-described combination including the two VGAs may be configured as below.
A first VGA amplifying a low-frequency signal and a second VGA amplifying a high-frequency signal obtained by up-converting the frequency of a signal output from the first VGA may be used in combination. Then, control is performed so that the gain of the first VGA is monotonously increased in steps of a single least-significant-bit (LSB) width, and control is performed so that the gain of the second VGA is gradually increased while being vibrated by as much as a single LSB width. Consequently, the entire gain control curve has a linear characteristic.
Further, a first VGA amplifying an intermediate-frequency signal, a mixer up-converting the frequency of an output of the first VGA, and a second VGA amplifying the high-frequency signal of an output of the mixer may be used in combination. Then, adjustments are made to obtain an intermediate-frequency signal having a distortion amount and a signal-to-noise ratio that are appropriate for the mixer when the gain amount of the first VGA corresponds to the maximum output level.
Further, a first VGA amplifying an intermediate-frequency signal and a second VGA amplifying a high-frequency signal obtained by up-converting the frequency of an output of the first VGA may be used in combination. Then, the gain of the first VGA is controlled so that the level of output power is changed over a lower range of the output power level, and the gain of the second VGA is controlled so that the output power level is changed over a higher range of the output power level.
Further, a VGA amplifying a high-frequency signal and a power amplifier amplifying an output of the VGA may be used in combination. At the small-output transmission time, the power amplifier is turned off and bypassed.
A high-frequency transmitter should be configured so that noise output power attained outside the signal band at the maximum-power output time is limited to less than a specified value so as to reduce an influence on a different system and/or an adjacent channel due to the specifications thereof. For example, it is specified that noise power away from the signal band by as much as X Hz becomes −30 dBm or less when an output of the signal band becomes 20 dBm. In that case, the power ratio of a signal to the noise power (the carrier to noise (CN) ratio) is expressed by the equation 20 dBm−(−30 dBm) =50 dB.
Under the above-described limitations imposed by the specifications, the amplitude of a baseband signal and/or a low-frequency signal is increased so that the ratio of signal power to noise power (the CN ratio) is sufficiently increased. Further, amplification is performed through a VGA provided in the post stage so that desired transmission power is achieved. In that case, the VGA provided in the preceding stage to amplify the baseband signal and/or the low-frequency signal should be configured so that the CN ratio is sufficiently increased even though the VGA is controlled to attain the minimum gain. This is because the CN ratio of an output should satisfy the standard even though the VGA provided in the post stage is controlled to attain the maximum gain at the same time as when the VGA provided in the preceding stage is controlled to attain the minimum gain and a total of the gains becomes the maximum gain area.
However, when the level of the minimum gain of the VGA provided in the preceding stage is increased to such an extent that the above-described standard is satisfied, the level of the maximum gain of the VGA provided in the preceding stage becomes higher than the above-described level. Consequently, when the VGA provided in the preceding stage is controlled to attain the maximum gain when the VGA provided in the post stage attains a low gain, a signal with a large amplitude is input to the VGA provided in the post stage, so that a signal output from the post-stage VGA attaining the low gain is distorted and the communication quality is deteriorated.